1. Field of the Invention
The invention relates to wastewater processing reactors, and in particular, to aerated fixed channel growth media reactors.
2. The Prior Art
Aerated wastewater treatment systems designed for small applications (less than 50,000 gallon daily capacity) generally involve an aeration treatment chamber or zone for injecting air into the wastewater, and a clarifier chamber of zone, a quiescent zone in which particles are allowed to settle out of the system. An example of such a treatment chamber is shown in Hansel. As can be seen, the aeration treatment zone is generally an empty chamber having several air release sites, usually located at the bottom of the chamber. An aerated treatment system treats wastewater through aerobic bacterial degradation of the waste materials present in wastewater or sewage. Aerobic bacterial metabolic degradation requires dissolved oxygen and hence, the release of air into the treatment chamber. Anoxic (oxygen free) degradation can also occur, and such is particularly efficient in removing undesired nitrates. In the Hansel system, waters in the aerobic treatment chamber are aerated, and in the process of aeration, mixing occurs, assisting in the transfer of oxygen into the wastewaters. Waters in the treatment chamber will eventually migrate to the clarifier zone. In the clarifier zone, no mixing occurs and the waters are calm, providing conditions to allow suspended solids to settle out of the clarifier zone to be returned to the treatment zone for further processing.
In the Hansel device, mixing and aeration occurs in a media free zone. The bacteria/microbes float freely in the treatment zone, having no surfaces (other than the container/clarifier sidewalls) on which to attach. While such free floating bacteria are effective in treating wastewaters, it is believed that more efficient treatment can be accomplished by providing a surface for bacterial and microbe attachment as in trickling type filtration systems, and directing the waters through the treatment media for treatment. Systems utilizing submerged growth media include that of U.S. Pat. No. 6,153,099 to Weis, et al; U.S. Pat. No. 5,156,742 to Struewing; U.S. Pat. No. 5,030,353 to Stuth; U.S. Pat. No. 5,200,081 to Stuth; U.S. Pat. No. 5,545,327 to Volland; and U.S. Pat. No. 5,308,479 to Iwai, et al, all incorporated by reference. In these systems, growth media is provided in the treatment or reactor chamber (such as the floating media balls in Stuth or the corrugated panels of Volland, and the cross flow media or vertical flow media manufactured by Brentwood Industries of Reading, Pa., also shown in U.S. Pat. No. 5,384,178 and U.S. Pat. No. 5,217,788, all incorporated by reference). Air lift or air release channels or draft tubes (airlift pumps) are provided through the media, such as in Struewing (reference 26), Iwai (reference 3P), Weis (reference 28), Stuth '754 (reference 12), and Stuth '081 (reference 8). Air may also be released on an external side of the media, such as shown in Volland. However, in these devices, oxygen is not directly transferred to the growing biomass on the growth media, but only indirectly and inefficiently through oxygen absorbed in wastewaters (dissolved oxygen) transferred during the air lift operation.
Another device addressing clogging of media fixed film base treatment is the device shown in U.S. Pat. No. 5,484,524 to MacLaren, et al (incorporated by reference). This device shows media disposed in a tank with a central media free core. An aspirator or air release site is positioned in the media free core, which induces a current in the tank, upward through the core, and then substantially downward through the media (See FIGS. 8 and 9). This device does not provide oxygen directly through the media, and hence, still suffers from clogging (See U.S. Pat. No. 6,105,593 to MacLaren, et al, describing a cleaning probe for the '524 device) and is not as efficient in providing oxygen directly to the growing biomass.
A device utilizing air dispersed through the fixed media is shown in U.S. Pat. No. 5,500,112 to McDonald. McDonald shows a series of chambers filled with media. Air is released under essentially the entire media bottom through a membrane covered panel at the tank bottom and consequently, there is no established circulation path through the media volume—upward flowing waters and downward following waters are intermixed throughout the media volume. Additionally, the McDonald device is a series of tanks substantially filled with media: the McDonald device lacks a media free treatment volume (a buffer zone or dilution zone). This lack results in the need for an excessive amount of media to effect treatment, making the McDonald device inefficient and uneconomic. Additionally, the lack of a dilution or buffer zone in each reactor chamber makes treatment inefficient. With no dilution zone, McDonald places the aeration panels on the floor of each reactor. The reactor floor is where sludge (fully digested waste materials) normally would be deposited by precipitation. The McDonald device forces sludge in all three reactor chambers to remain in suspension until the sludge can be directed to a quiescent zone, the remote McDonald 4th chamber. However, access from one reactor to the next and eventually to the 4th zone, is through the fluid channels at the very top of the reactor, also tending to keep sludge, which would normally participate, in suspension in each reactor chamber. Consequently, McDonald each reactor chamber will have higher sludge concentration levels than in systems having a dilution zone. With higher concentration of solution sludge, treatment is more inefficient as the ration of usable (digestible) waste materials to total waste materials is suppressed.
In aerated growth media reactors, current flow in the system is induced by air injection. The induced current within the media is generally an upward flow through the air lift tubes (or in the case of Volland, on the side of the growth media) and downward through the fixed media. In aerated growth media treatment systems, waters remote from the treatment media must also be transported to the media surfaces for treatment, as treatment is substantially localized in the growth media. Hence, efficient mixing throughout the entire chamber is highly desirable. The use of air lift tubes generally induces a current in the treatment center sufficient to provide the needed full system mixing, that is, to bring waters remote from the growth media to the growth media for contact and treatment by bacterial colonies attached to the growth media.
Use of air lift tubes thus induces a current and provides indirect oxygen to the biomass. Air lift tubes also present scouring of the growth surfaces caused by rising bubbles interacting against the growth surfaces. As the introduced air is not passing upwardly through the growth media, upward turbulence through the growth media is reduced. Reduced upward turbulence in the growth media increases the potential for bacterial growth to occlude the channels, thereby plugging or clogging the flow channels in the media. One attempt to minimize plugging is shown by Volland. Volland uses corrugated panels placed back to back creating channels orientated at 60 degrees from the vertical. Volland thus tries to direct the bacterial slough-off down the channels to the bottom of the media.
Growth media treatment systems as shown additionally introduce wastewaters into the growth media by pumping incoming wastewaters into a portion of the system remote from the media, and allowing the induced current to transport the new influx of treatable materials to the treatment media. This process, however, dilutes the raw incoming sewage or wastewater and extends the time for materials present in the incoming waters to be transported to the treatment media.
Finally, all small plant treatment systems in the United States must pass stringent regulatory requirements for effluent quality and plant performance. Two of the plant performance characteristics that fixed growth media treatment plants have difficulty achieving are start-up time and vacation time. These are time requirements during which a plant must meet effluent standards: start-up time refers to the time a newly installed plant must meet effluent standards after initial start-up; vacation time refers to the time a plant must meet effluent standards after re-starting from a dormant period (a vacation). The regulatory requirement for start-up/vacation times are difficult to achieve for growth media surfaces as the biomass on the growth surfaces must either be established (for start-up) or replenished after a period of starvation (during the dormant period). The biomass response time to condition changes, when localized as on a growth media, is generally slower then in the extended aeration system, such as the Hansel system. Consequently, a growth media treatment system will take longer to start-up than an extended air treatment system.